Discovery of a New Haptic Illusion: Perceived Speed Varies Despite Constant Actual Speed
A world-first achievement leading to richer haptic experiences in XR

News Highlights:

Figure 1 Discovery of Haptic Illusion Where the Perceived Speed of a Moving Object on the Skin Changes Based on Spatial Interval
Note. Even when an object moves at the same speed, the perceived speed is affected by the spatial interval used to present the motion. When the spatial interval is larger, the motion feels slower, while a smaller spatial interval makes the motion feel faster.

Background

At the NTT Communication Science Laboratories, we have been conducting basic research in human science to deeply understand human sensory and perceptual mechanisms. In the past, we have utilized illusions to elucidate the mechanisms, and based on these findings, we have created world-first high-quality sensory experiences^1,2. In this study, we discovered a new haptic illusion related to the perceived speed of motion through touch (Figure 1). This finding has the potential to contribute to the realization of rich haptic experiences in XR.
　In our daily lives, we engage in various haptic experiences, such as skillfully manipulating objects or discriminating multiple objects via touch. These haptic experiences are supported by basic sensory and perceptual functions, such as the perception of texture, shape, and motion. In this study, we focused specifically on the perception of motion.
　Previous research has investigated perceived speed³. However, these studies were conducted using physical objects, where multiple cues (such as skin shear deformation, vibration, and positional movement cues) were mixed and not clearly separated (Figure 2). To address this, we developed a specialized haptic interface to precisely study human perceived speed by isolating these cues. This interface allows for wide-area, high-density information presentation⁴. By sequentially presenting information at discrete points on the skin, we were able to provide positional movement cues alone. Using this specialized interface, we investigated the perceived speed of motion.
　In practical applications, such as XR, haptic interfaces typically convey motion to users generally using only positional movement cues. Therefore, the findings gained from this study, which focuses on positional movement cues alone, hold significant relevance for such applications.

Figure 2 Positioning of This Study
Note. Previous studies investigating the perceived speed of motion mainly used physical objects, where multiple cues were mixed and not separated. In this study, we used a developed haptic interface to investigate the perceived speed based solely on the cue of positional movement.

Overview of the Results

In this study, we first developed a specialized haptic interface capable of wide-area, high-density information presentation, which is essential for investigating perceived speed. Using this interface, we conducted experiments to study the perceived speed of motion and discovered, for the first time, a haptic illusion where the perceived speed changes based on the spatial interval.

(1) Development of a Specialized Haptic Interface for Wide-Area, High-Density Information Presentation

To study perceived speed, it was necessary to flexibly manipulate the information presented across a wide area of the skin. We thus expanded a previously developed haptic interface which could present high-density information on the fingertip⁵ to one covering the entire hand. In typical haptic interfaces, the actuation and presentation components are spatially integrated, making it difficult to build an interface capable of wide-area, high-density information presentation. In contrast, the developed interface, composed of separate pneumatic actuation and presentation components, enables wide-area (from fingertip to palm) and high-density (3mm intervals) information presentation (Figure 3). This interface allows for flexible manipulation of parameters such as spatial intervals for information presentation across a wide area.

Figure 3 Experimental Haptic Interface for Wide-Area, High-Density Information Presentation
Note. This specialized haptic interface allows flexible manipulation of parameters such as the spatial interval of information presentation.

(2) Discovery of a Haptic Illusion on Perceived Speed of Motion

Through experiments using this haptic interface, we discovered a new haptic illusion on the perceived speed of motion. In the experiments, participants were presented with a virtual motion of an object moving across the surface of their skin — from the fingertip to the palm — by sequentially stimulating discrete points on the skin (Figure 4A). Participants were presented with two types of motion (a reference motion and a comparison motion) and asked which one felt faster. The spatial interval between the discrete points in the reference motion was set to either 3 mm or 6 mm (Figure 4B), while the interval in the comparison motion was fixed at 3 mm. Based on participants' responses, we determined the speed of the comparison motion that felt as fast as the reference motion, allowing us to quantify the perceived speed of the reference motion and analyze how spatial intervals influenced it.
　Since the spatial interval between the discrete points corresponds to the spatial resolution of the information presented — and can be manipulated independently of the motion's actual speed — it was previously believed that spatial interval would not affect the perceived speed of motion. However, the experiment revealed that the larger the spatial interval, the slower the motion felt (Figure 4C). Specifically, when the spatial interval doubled, the motion was perceived to be about 0.75 times slower.

Figure 4 Experimental Conditions and Results
Note. (A) The motion presented to participants in the experiment. A virtual constant-speed motion of an object was conveyed from the fingertip to the palm by discretely moving the contact points on the skin. (B) The spatial interval conditions for information presentation. (C) Experimental results.

Outlook

The result of this study suggests that by carefully designing how motion is presented through touch, with attention to spatial intervals, it may be possible to better control how fast movement feels in general haptic interfaces. We plan to explore other basic sensory and perceptual functions that support our haptic experiences, such as shape perception, to help create entirely new haptic experiences in XR.

[Publication Information]

Journal Name: iScience
Title: Spatiotemporal motion features resulting from tactile interface layouts influence tactile speed perception
Authors: Yusuke Ujitoko, Yuko Takenaka, and Koichi Hirota
DOI: https://doi.org/10.1016/j.isci.2024.110803

[Glossary]

^1.Press release： "World's first: A display system to present a large-scale 3D aerial image beyond the frame of multiple monitors using illusion" https://group.ntt/en/newsrelease/2024/06/17/240617d.html

^2.Press release： "Discovery of a New Principle for Presenting Tactile Sensations Using Haptic Illusion Phenomena" https://group.ntt/jp/newsrelease/2021/05/31/210531b.html (Japanese)

^3.Perceived Speed: The magnitude at which a human perceives the speed of an object's movement.

^4.Information Presentation: Providing physical contact to the skin.

^5.Ujitoko, Y., Taniguchi, T., Sakurai, S., & Hirota, K. (2020). Development of finger-mounted high-density pin-array haptic display. IEEE Access, 8, 145107-145114.

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